Why is the inner core a solid even though it has a higher temperature than the outer core

Why is the inner core solid even though it is hotter than the outer core?

Question Date: 2001-10-15

Answer 1:

The inner core is indeed hotter than the outer core. However, the PRESSURE on the inner core is greater than the pressure on the outer core and the melting point of iron, the main constituent of the core, INCREASES as the pressure goes up. So, because the pressure effect overrides the temperature effect, the inner core is solidified.

We also can speculate that the inner core began to freeze (solidify) about 1 or two billion years ago. the inner core makes up only 1% of the mass of the Earth; the outer core makes up about 32% of the mass of the Earth.

Answer 2:

The inner core is solid because it is made of very dense, or heavy, materials - like iron and nickel. Even though it is very hot, these materials don't "melt" very easily, so they stay solid.

Answer 3:

It turns out that many materials can be a solid at a higher temperature if the pressure is also higher. So, even though it is hotter in the inner core, the pressure in the core is also higher, and you can have solid iron-nickel instead of liquid.

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I have never understood why earth's inner core is solid. Considering that the inner core is made of an iron-nickel alloy (melting point around 1350 C to 1600 C) and the temperature of the inner core is approximately 5430 C (about the temperature of the surface of the sun). Since Earth's core is nearly 3-4 times the melting point of iron-nickel alloys how can it possibly be solid?

asked Apr 24, 2014 at 16:03

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Earth's inner core is solid even though the temperature is so high because the pressure is also very high. According to the Wikipedia article on the Earth's inner core, the temperature at the center is $5,700\ \text{K}$ and the pressure is estimated to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$).

The phase diagram shown below (taken from this paper) shows the liquid/solid transition, where fcc and hcp are two different crystalline forms of solid iron. You can see clearly from the slope of the line going off toward the upper right that iron should be solid at this temperature and pressure.

answered Apr 24, 2014 at 16:18

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In addition to the answers below and my comment above, I believe the following phase diagram, from DavePhd's answer here, sourced from here, is more appropriate for the pressure levels near the Earth's core of about 330 to 360GPa.

We can see from the image that for pressures between 330 and 360GPa, the melting temperature ranges from about 6200 K to 6600 K, which is much higher than Earth's inner core temperature of about 5700 K.

answered Apr 25, 2014 at 4:53

KenshinKenshin

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You are only considering the melting point at atmospheric pressure. Melting point depends upon pressure. The pressure in Earth's core is about 350 GigaPascals. It is important to study the phase diagram of the substance being considered. A phase diagram explains what phase (solid, liquid, gas) a substance is in at various temperatures and pressures.

See Phase diagram of iron, revised-core temperatures for detailed information.

answered Apr 24, 2014 at 16:16

DavePhDDavePhD

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